The present invention relates to an active material used in a positive electrode for alkaline storage batteries such as nickel-cadmium storage battery, nickel-metal hydride storage battery and the like. More particularly, it relates to an active material comprising a nickel based multi-metals oxide.
Recently, an alkaline storage battery, particularly portable sealed storage battery, has widely been used as a main power source for various portable apparatuses such as communications equipment, business machine, electrical appliance, miscellaneous goods, etc. because it is superior in well-balanced charge/discharge characteristics, cycle life and safety/reliability to other batteries. Also, it has attracted special interest as a large power source, e.g. movable main power source for electric vehicles, etc. because it is extremely superior in charge/discharge characteristic and reliability.
A typical alkaline storage battery is a nickel-cadmium storage battery with a long history. A nickel-metal hydride storage battery using a metal hydride in place of a cadmium negative electrode of this battery has recently been industrialized and a share thereof has rapidly been increased.
In order to improve the energy density and reliability, as in the past, the followings have become extremely important, that is, (1) means for filling a large amount of active materials of positive and negative electrodes in a predetermined volume by realizing light-weight, thin volume, short length and small size of a substrate and additives in an electrode, a separator, an electrolyte, a battery case and a lid member, (2) improvement of various additives and conductive materials, which enhance utilization of an active material, and (3) development of a novel active materials which exhibits high energy density under various use conditions.
Therefore, a recent technical tendency with respect to them will be described hereinafter.
As a main active material of a positive electrode in the industrial nickel-cadmium storage battery and nickel-metal hydride storage battery, a nickel oxide (NiOOH) has hitherto been used. However, as a substrate of the electrode, a network substrate having a higher porosity (e.g. foamed nickel substrate, etc.) has recently been applied in place of a sintered plaque which has been used in a conventional high performance, long cycle life sintered electrode, although the network substrate has a three-dimensional construction. As a result, an electrode wherein the foamed nickel substrate is filled with a large amount of an active material powder (hereinafter referred to as a "foamed metal type electrode") was industrialized, so that the energy density of the nickel electrode was drastically improved (U.S. Pat. No. 4,251,603). An electrode using as a substrate a felt of nickel having the same feature as that of the foamed nickel substrate is also known.
A common advantage of using such high porosity substrate is that a simple producing method capable of directly filling a nickel oxide in the form of paste in the substrate can be used because a pore diameter can be increased unlike the conventional porous sintered substrate. On the other hand, there arose a problem that, since a powder having a large particle diameter is filled in a substrate having a pore diameter larger by far than that of the sintered substrate, influences of low conductivity of the active material powder and decrease in electrical conductivity between the active material and the substrate as a current collector are remarkably exerted, which results in deterioration of the utilization of the active material. Therefore, the conductivity has been compensated by using a method of adding Co or an oxide thereof, Ni, etc., in addition to the active material powder, that is, nickel oxide powder, or still insufficient conductivity has been compensated by incorporating metallic elements other than Ni, such as Co, etc. into the nickel oxide to form a solid solution.
It has been found that the incorporation of other metallic elements into the nickel oxide also results in remarkable improvement in charge efficiency, and incorporation of two elements Co and Cd has a remarkable effect, particularly. Thereafter, Zn having a property which is similar to that of Cd is noted and used as a substitute element for Cd and, furthermore, a solid solution material with three elements Co, Zn and Ba incorporated therein is suggested. The incorporation of other elements into the nickel oxide for the purpose of realizing high efficiency of the charge/discharge characteristic is a technique which has been known for a long time in the sintered electrode. A modification of using a solid solution nickel oxide incorporated with one or more elements selected from Mg, Ca, Ba, Ti, Zr, Mn, Co, Fe, Cu, Sc, Y, etc. is exemplified.
The incorporation of the element such as Co, Cd, Zn, etc. into the nickel oxide has an inhibitory effect on the formation of a highly oxidized compound, i.e. nickel oxyhydroxide of a .gamma. phase during overcharge, in addition to an effect of improving the charge acceptance. Therefore, the incorporation of the above metallic elements was an effective means for realizing long cycle life in the case of applying to a fragile foamed metal type electrode, unlike a fast sintered electrode because volume swelling of the nickel oxide is inhibited (U.S. Pat. No. 5,366,831).
In addition to the improvement of the active material, a shape of the active material is also improved and formed into a spherical shape which is suitable for high density filling and, therefore, it has become possible to use the active material in a practical battery.
The above method of adding Co or an oxide thereof is further improved, and a method of forming a coating layer of Co(OH).sub.2 on the surface of the active material powder or a method of forming a powder layer of a Co oxide has been suggested. These methods aim to realize higher efficiency of the utilization of the active material and to improve the productivity by improving the efficiency of a method of adding a conductive agent.
With the development of these techniques, the charge/discharge efficiency of the active material powder filled in a density which is higher by far than that by a conventional technique can be enhanced to the same level of an excellent sintered electrode. Therefore, the energy density of the positive electrode is remarkably increased and a nickel positive electrode having an energy density of about 600 mAh/cm.sup.3 is put into practice at present.
On the other hand, with respect to a negative electrode, the energy density was largely improved by applying a metal hydride (AB.sub.5 system) having high capacity density in place of a conventional cadmium negative electrode and, therefore, a negative electrode (per unit volume) having at least twice as much energy density as a positive electrode has been put into practice. In response to this, thinning of a separator, a battery case and other parts has rapidly advanced and the energy density of the battery has been increased.
However, as described above, a demand for improvement in energy density of the battery as a power source for portable apparatus has become greater and greater. In order to realize further improvement in energy density of the battery in response to such a demand, it is strongly required to realize higher energy density and higher performance of the positive electrode related to the development of a technique of realizing higher energy density of the negative electrode.
Furthermore, in view of the recent use, it is further required strongly to realize high energy density, long cycle life and safety at high temperature within the range wider than that in the case of a conventional use, particularly from about 45 to 60.degree. C., with the variation of the use conditions of a portable electronic equipment applied as a power source. The same may be said of a large type movable main power source to which realization of small size and light-weight is required in a severe operating atmosphere.
In a foamed metal type electrode or felt type electrode having an energy density higher than that of the sintered electrode, there is a limit in the restudy of a reduction in metal amount of the substrate and the kind and amount of additives and the filling density of the active material has almost reached the limit. As is generally said, the utilization of the active material in the case of assuming that one electron reaction of Ni is utilized has almost (100%) reached the limit and, therefore, rapid realization of higher energy density can not be desired as the matter stands. From these points of view, in order to realize higher capacity density and higher performance, not only the restudy of the substrate and additives but also the development of the active material itself having epoch-making high energy density are required.
The current active material will be explained in more detail hereinafter. As described above, a material composed mainly of a nickel oxide (Ni(OH).sub.2) is used at present as a positive electrode material of an alkaline storage battery which is industrially used. It is considered that the reaction is mainly one electron reaction between 2 valent and 3 valent of Ni in .beta.-type crystals, as shown below. ##STR1##
However, in the actual battery, the reaction between about 2.2 valent and about 3.2 valent in an average value may occur (in this case, it is often referred to as a reaction between .beta.-Ni(OH).sub.2 and .beta.-NiOOH). Anyway, it is a reaction corresponding to approximately one electron. With respect to .beta.-NiOOH in charged state, when charging is conducted under low-temperature atmosphere or charging is conducted for a long period of time, or overcharging is repeated, a part thereof is oxidized to form .gamma.-NiOOH having a higher Ni oxidation state than .beta.-NiOOH. When it is oxidized to form .gamma.-NiOOH, the volume swells and, therefore, the electrode is liable to swell. .gamma.-NiOOH is an electrochemically inert material. Therefore, when .gamma.-NiOOH is formed, there arises a problem that the capacity is reduced and the voltage of the battery is lowered as a result of an increase in overvoltage. Accordingly, a trial of inhibiting the formation of .gamma.-NiOOH has hitherto been made.
Incidentally, it has hitherto been considered that .gamma.-NiOOH is represented by about 3.5 to 3.8 valent of Ni, specifically the chemical formula A.sub.x H.sub.y NiO.sub.2.nH.sub.2 O (an alkali metal A is intercalated between layers composed of Ni and O, thereby to balance a charge between A, H, Ni and O). It is also considered that the valence of Ni is 3.67 or 3.75 and .gamma.-NiOOH is known as a nonstoichiometric compound.
In order to realize higher energy density by using a nickel oxide-based material as the active material of a secondary battery, it is extremely important to make good use of this .gamma.-NiOOH phase, in other words, to find out a material of more than one electron reaction. Therefore, there is a report describing a method of directly forming an .alpha.-Ni(OH).sub.2 phase having a wide interlayer distance (this is in the discharged state, but the interlayer distance is about 8 angstroms close to about 7 angstroms as the interlayer distance of .gamma.-NiOOH in the charged state) other than the .beta.-Ni(OH).sub.2 phase (most adjacent interlayer distance: about 4.6 angstroms) by incorporating a trivalent metal (e.g. Al.sup.3+, Fe.sup.3+, etc.) into a nickel oxide, thereby to form positively charged metal oxide layers, and then incorporating an anion in the metal oxide layers so as to totally balance a charge (e.g. U.S. Pat. No. 5,348,822, etc.).
Such an oxide easily forms .gamma.-NiOOH phase by charging due to the small difference of interlayer distances. Thus in the .alpha.-Ni(OH).sub.2 /.gamma.-NiOOH reaction system a larger number of electrons per nickel atom are transferred than in the .beta.-Ni(OH).sub.2 /.beta.-NiOOH reaction system. However, there arises a problem that the density of the material itself (material density, i.e. tap density) is drastically lowered because of the presence of the .alpha.-Ni(OH).sub.2 whose interlayer distance is much wider than that of .beta.-Ni(OH).sub.2. Since the tap density has a positive correlation with the filling density upon production of the electrode, high density filling becomes very difficult.
By using this material, there arises a new problem that the cycle life becomes short and the discharge voltage is lowered.
Furthermore, points to which special attention should be paid are as follows. That is, a problem of deterioration of the charge efficiency of the Ni-based oxide at high temperature is not solved at all only by using the above material. High-temperature characteristic is a performance which has recently been regarded as particularly important with the diversification of the use conditions of the battery, and is an object which can not be ignored in view of the utility in the future development of the secondary battery.
The charge efficiency is lowered at high temperature because, when the charge potential of the Ni-based oxide approaches the oxygen evolution potential, oxygen is liable to be evolved in the terminal stage of the charge as a result of the competitive reaction with the charge reaction. Accordingly, there is a suggestion of incorporating or adding an element for increasing the oxygen evolution overvoltage even at high temperature into the nickel oxide in a mediate/large nickel-metal hydride storage battery which easily becomes high temperature in order to improve the charge efficiency at high temperature (U.S. Pat. No. 5,455,125). However, this suggestion was not rooted in a way of thinking about the improvement of the electrode energy density, considering the use of the .gamma.-NiOOH phase. That is, it aimed at the improvement in charge efficiency at high temperature in the charge/discharge reaction between .beta.-Ni(OH).sub.2 phase and .beta.-NiOOH phase.
The incorporation of the element for increasing the oxygen evolution overvoltage had a drawback that the content of Ni, which mainly supports the charge/discharge reaction, is reduced. Accordingly, it was not insufficient, taking the improvement in energy density into consideration together.
In summary, in order to provide a high energy density, high performance positive electrode for an alkaline storage battery, a novel active material capable of realizing an energy density higher by far than that of a conventional one under various use conditions, particularly use at high temperature, that is, high utilization under high density filling is required. Therefore, it is necessary to improve the following problems.
(1) The material must be a high utilization active material more than one electron reaction in the charge/discharge reaction. That is, with respect to the Ni-based oxide, a higher oxidized compound .gamma.-NiOOH (the oxidation state of Ni is from 3.5 to about 3.8) instead of .beta.-NiOOH (the oxidation state of Ni is about 3.0) of a conventional material should be used as an active material in the charged state.
(2) The active material should be a material which is suitable for high density filling at production of the electrode.
(3) The charge/discharge reaction should be conducted in an efficiency higher than that of a conventional material even at high temperature.
(4) The discharge voltage should be the same as or higher than that of a conventional material even at high rate discharge.
It is important to simultaneously solve all of the above problems. That is, it is necessary to develop a Ni-based oxide capable of filling an active material which exhibits high utilization in high density, and a material having high charge/discharge efficiency.